The present invention relates to voltage level detecting and indicating circuit apparatus in general and in particular to a MOS voltage level detecting and indicating circuit apparatus for detecting and indicating a voltage level in a MOS integrated circuit.
MOS circuits are widely used in small portable electronic calculating apparatus which utilize light-emitting diodes (LED's) for output display. To insure against marginal operation, the calculators are typically provided with a low supply voltage indicator. However, because of the current requirements for driving a LED display, it had been the practice to include as an interface between the MOS calculator chip and the LED display a bipolar LED digit driver circuit, and because of a lack, heretofore, of suitable MOS voltage level detecting apparatus, the digit driver circuit typically included a voltage level detecting and indicating circuit which was coupled to at least one element of the display for providing a visual indication when the magnitude of the supply voltage (i.e., battery) became less than a predetermined level below which the calculator would not operate reliably.
Among the disadvantages of using a separate bipolar digit driver and low voltage indicating circuit are the cost of a separate integrated circuit, the space required to house the circuit and the fixed level of the supply voltage at which the bipolar voltage level detecting and indicating portion of the driver circuit operates. Because of the fixed operating level of the prior known voltage level detecting and indicating circuits as used in the bipolar digit driver circuits, manufacturers of MOS electronic calculators have, heretofore, generally been required to discard all MOS chips which would not operate or would become marginally operable at voltage supplies below the fixed level.
Recently MOS circuits capable of handling the higher current requirements of LED circuits have become economical. With the advent of these higher current handling capabilities using MOS devices, it becomes desirable to eliminate the bipolar digit driver circuit altogether including the voltage level detecting and indicating circuit with which it has been associated. In doing so, various considerations, including the dependability of voltage levels on process variables, become important.